"A dark object may be lurking near our solar system, occasionally kicking comets in our direction. Nicknamed 'Nemesis' or 'The Death Star,' this undetected object could be a red or brown dwarf star, or an even darker presence several times the mass of Jupiter. Why do scientists think something could be hidden beyond the edge of our solar system? Originally, Nemesis was suggested as a way to explain a cycle of mass extinctions on Earth."
Pretty interesting stuff. It'll be interesting to see the results of tests supporting or disproving its existence.